Recently, devices such as a mobile phone that uses large-capacity wireless communication have been spread. In such wireless communication, it is necessary to use an omnidirectional antenna capable of isotropically transmitting a radio wave as a base station antenna in order not to limit a position of a mobile terminal with respect to a base station.
As an example of such antenna, an omnidirectional antenna that can prevent a deviation from a maximum radiation direction position in a vertical plane directionality and can suppress a level deviation in a horizontal plane directionality, has been proposed (Patent Literature 1). FIG. 15 is a front view illustrating a configuration of an antenna 700 according to Patent Literature 1. FIG. 16 is a back view illustrating the configuration of the antenna 700 according to Patent Literature 1.
The antenna 700 includes half wavelength dipole antenna elements 710A and 710B. The dipole antenna elements 710A and 710B are vertically arranged in such a manner that longitudinal axes of those are aligned in a vertical line and the dipole antenna elements 710A and 710B are not to contact each other. A coaxial cable 740 can be inserted between the upper dipole antenna element 710A and the lower dipole antenna element 710B. The element conductors 711 and 712 constituting the dipole antenna elements 710A and 710B are formed of a metal foil adhered to a dielectric substrate 720. The element conductor 711 is formed on a top face of the dielectric substrate 720 and the element conductor 712 is formed on a back face of the dielectric substrate 720.
On the dielectric substrate 720, a dual-distribution feed line 730 is formed to be parallel to the longitudinal axis of the dipole antenna elements 710A and 710B. The dual-distribution feed line 730 includes a conductor line 731 formed on the top face of the dielectric substrate 720 and the conductor line 732 formed on the back face of the dielectric substrate 720 to face the conductor line 731. The dual-distribution feed line 730 is arranged at a position laterally away from the longitudinal axis of the dipole antenna elements 710A and 710B (on a right side in FIG. 15) by a predetermined distance. An upper end and lower end of the conductor line 731 are each connected to the element conductors 711 of the dipole antenna elements 710A and 710B. An upper end and lower end of the conductor line 732 are each connected to the element conductors 712 of the dipole antenna elements 710A and 710B
The coaxial cable 740 serving as a main feed line is closely disposed on the top face of the dielectric substrate 720. A core conductor of the coaxial cable 740 is connected to a branch point of the conductor line 731 and an outer conductor of the coaxial cable 740 is connected to a branch point of the conductor line 732. The coaxial cable 740 passes between the element conductor 712 of the dipole antenna element 710A and the element conductor 711 of the dipole antenna element 710B and then is guided downward to be parallel to the longitudinal central axis of the dipole antenna element 710B. In other words, the coaxial cable 740 is disposed in such a manner that a part of the coaxial cable 740 guided downward is located on a left side of the dipole antenna element 710B in FIG. 15. A distance between the longitudinal central axis of the dipole antenna element 710B and the coaxial cable 740 substantially coincides with a distance between the longitudinal central axis of the dipole antenna element 710B and the dual-distribution feed line 730. Therefore, the coaxial cable 740 and the dual-distribution feed line 730 are positioned substantially symmetrically with respect to the dipole antenna element 710B that is a center.
In the antenna 700, the dipole antenna elements 710A and 710B each radiate radio waves that are omnidirectional in the horizontal plane and the dual-distribution feed line 730 and the coaxial cable 740 in the vicinity of the dipole antenna elements 710A and 710B function as a reflective conductor. Since the dual-distribution feed line 730 and the coaxial cable 740 are positioned substantially symmetrically with respect to the dipole antenna element 710B that is the center, deteriorations of radiation level due to reflective functions of the dual-distribution feed line 730 and the coaxial cable 740 are cancelled. Therefore, a level deviation that is a difference between the maximum radiation power level and the minimum radiation power level decreased.
Besides, for a patch array antenna, a parallel power feeding method for an antenna element, which can realize miniaturization and a wide bandwidth, has been proposed (Patent Literature 2).